Does anybody recognize the following problem, and most important, does anybody know what to do...?

I have a Wacom Ultrapad A5. (PS 5.5, Latest Wacom drivers and Windows 2000).
When I use the paintbrush tool with pressure sensitivity on, it is mostly ok.
(For example black on white background).
But sometimes I get these black lines, like the sensitivity was turned off. Then it goes back to normal again. Of course I can undo the last actions, but it gets kinda frustrated after a while. I had the same problems with Photoshop 4 and Win95. Hope you can help me....
http://go.to/penk